1. Field of the Invention
The present invention relates generally to a magnetic recording head and, more particularly, to a matrix-type thin film magnetic recording head for recording of multiple-tracks on recording tapes, for example.
2. Description of the Related Art
To keep pace with the ever increasing performance requirements of magnetic storage devices (for example, hard-disks, tapes drives or floppies), ever increasing area densities and data rates are required. The area density is limited by lateral tape motions, residual head servo tracking errors, mechanical tolerances on the head, thermal and hygroscopic coefficient of expansion of the media, etc. The most widely used approach to achieve both high area densities and high data rates is to write and read simultaneously several tracks in parallel on the tape. In standard tape drives using multiple heads, the minimum distance which is possible between neighboring heads (due mostly to the pitch of the excitation coil) still leads to a large distance between the outermost tracks which are written simultaneously when a high degree of parallelism is required (using several tape heads). The tape is filled by sequential interleaved writing, where neighboring tracks are written at different times. This interleave writing can be avoided by writing adjacent tracks at the same time using matrix arrays of heads in which packs of adjacent tracks with no or little guard band between tracks can be written simultaneously.
Such matrix heads can be made mechanically by several steps of sawing, winding and bonding, such as is known from the prior art, for example as in the fabrication of Metal-In-Gap (MIG) heads. It can also be made by hybrid technologies combining the above mentioned techniques and thin film technologies as described in the French patents Nos. 2 630 853 and 2 648 940. Narrow and densely packed data tracks can however be advantageously obtained by using full thin film technology. In thin films technologies, however, the manufacturing cost of the head is directly proportional to its surface area on the wafer. To decrease the footprint of the head on the wafer, it is necessary to minimize the area lost for the electrical connections and the pitch between adjacent heads, hence reducing the size of an elemental head. The minimization of the lost area can be achieved via interconnects, as described in the U.S. Pat. No. 5,933,940, which pushes the connection pads to the back side of the wafer, e.g. the side opposite to the recording media. The minimization of the pitch has been described in the U.S. Pat. No. 5,124,869. In the apparatus described therein, the magnetic field of the write head is not produced by a coil wound around part of the magnetic circuit, but by two single row and column conductors crossing under the magnetic poles of the corresponding write head. These column and row conductors are aligned with a head matrix network so as to obtain one head at each row/column intersection. The fabrication technique is generally of the hybrid type, using a magnetic (preferably ferrite) grooved substrate onto which the conductors are arranged and magnetic poles and gaps to close the magnetic circuit obtained either by thin layer deposition or from a second substrate which is subsequently bonded to the first substrate.
Although this head provides for a great integration, in particular due to the absence of coil windings, it has several drawbacks.
First: the addressing of a single head element is done by the superposition of the row and column excitation field, the field created at the gap of the head onto the medium being practically proportional to the sum of the excitation current of the row and column. The head has therefore to be designed such that an individual excitation by a given row (or column) is insufficient to overcome the medium coercive field, whereas the sum of the row and column excitation contributions is large enough to overcome the coercive field. In the original embodiment, this is done by using a dipolar pulse of amplitude 2I/3 on the columns (or row) and a pulse of amplitude +I/3 or xe2x88x92I/3 on a row (or column). These recording embodiments however can lead to a significant parasitic recording signal being sent by a neighboring cell on the corresponding column, in the form of an undefined magnetic state on the medium. This is known as crosstalk, in other words, recording by a head which has not been selected to write. To overcome this issue, complex recording schemes have been developed such as described in the U.S. Pat. No. 5,394,286. Other approaches include using saturable soft magnetic layers within the gap which would saturate (and hence allow the magnetic flux to leak onto the medium only if the excitation field is large enough), a writing scheme by inhibiting a 4 pole magnetic head, secondary gaps to increase the head overall reluctance, a derived current of opposite polarity to the excitation current in neighboring cells through a well chosen resistor, addressing ICs, etc. These refinements are described in particular in the U.S. Pat. Nos. 5,973,890; 5,063,467; 5,546,255; 5,086,362, the published International PCT Patent Application No. W094/15332, and the European Patent Document EP0 463 908. All add some undesirable complexity to the system.
Second, as linear excitation conductors that correspond basically to a single turn coil (two turns if one considers the sum of a row and column conductor for the selected head), the excitation current required to achieve the gap field which can reverse the media magnetization is rather high. An alternative in which the excitation is provided by an independent magnetic circuit using a multiple turns coil coupled to single conductor that is in to turn coupled to the write head has been described in the U.S. Pat. No. 5,671,106. The major drawbacks of this design, however, are a large total footprint of the head on the wafer (and hence a high cost), and in a matrix design an increased inductive/capacitive crosstalk through the connection conductors.
Third: manufacturing which involves both macroscopic and thin film technologies does not allow for an increased integration for future generation products which will require ever decreasing individual data track widths. The integration into a full thin film technology has been described in the U.S. Pat. No. 5,933,940 based on the row/column excitation conductor design.
The present invention enables all of the above mentioned limitations to be overcome, using a matrix array of adjacent and independently controlled write heads using only thin film technologies. Furthermore, the present invention apparatus allows for the head surface contour (tape bearing surface) to be integrated during the manufacturing process.
The device of the present invention includes a matrix array of recording heads, wherein each head is independent from another both in terms of its magnetic circuit and its excitation circuit. In one embodiment, the matrix array is fabricated using thin film technology and a planar design, as described for example in the patent U.S. Pat. No. 5,863,450, filed by one of the applicants and incorporated herein by reference. In another embodiment, the heads are aligned in an oblique lattice with the write gaps aligned along a (horizontal) row and offset along a (vertical) column by a constant value.
Each magnetic head includes a magnetic circuit formed by a bottom pole piece, two pillars and a top pole piece cut by a non-magnetic (generally insulating) gap. A conducting coil is wrapped around either the pillars or the bottom pole piece to provide, when excited by an electrical current, a magnetic flux within the magnetic circuit which magnetizes the recording medium in the vicinity of the gap where the flux leaks out in space. Preferably, the magnetic heads are of a helical type, for example, wrapped around the bottom pole piece to ensure a better coupling with the magnetic circuit, hence a lower excitation current or a lower number of turns, and a smaller footprint. To fabricate these helical heads, it is best to use the thin film technology in which the different elements are shaped by successive steps of thin film deposition, photolithography, etching, with intermediate steps of encapsulation and planarization. The heads are fabricated onto a non-magnetic substrate, preferably silicon, which acts as the head body.
The gap of the upper poles, which is a critical part of the magnetic circuit, is fabricated using conformal deposition. This techniques uses the following sequence: fabrication of one of the poles, conformal deposition of the gap, fabrication of the second pole, and planarization to bare the gap. With a method of this type, as already proposed in the patents U.S. Pat. No. 4,942,490 and International application No. W099/67777, it is possible to obtain very thin gaps (hence large writing fields) at the expense of an oblique gap and a residual gap under the first pole piece, hence an unnecessary high reluctance of the magnetic circuit. The process used in the present invention differs markedly for the above in that it allows a vertical gap with no residual gap material under any of the pole pieces.
Each of the individual excitation coils is connected to the back side of the substrate by interconnects through the non-magnetic wafer. Preferably, the interconnects are shifted to the sides of the matrix array itself to allow the smallest pitch between each individual head. The interconnects can be made by drilling into the silicon and filling with a conducting material, by using highly doped conductive silicon with etched trenches, or any other method thereof. On the side opposite to the heads the interconnects can advantageously be terminated by low melting temperature bumps such as TiSn or SnPb or any other material, in order to allow for the connection of the die to the control electronics, for instance through a flex cable. In a preferred embodiment, two or more heads have one common lead so as to decrease the number of interconnects.
The individual heads are arranged in rows and columns in order to simultaneously write parallel tracks of data onto the media. The general layout is defined by external parameters such as the overall size of the matrix array, the number of tracks to be written, the data track width and the track density, etc. In a preferred embodiment, the write gaps within a given column (row) are offset laterally one from another but they remain aligned horizontally along a row (column). All the rows (or columns) are then parallel one to each other but the overall matrix array is not necessarily square. This allows the writing of adjacent tracks without the need for tilting the tape with respect to the head or vice versa. In another preferred embodiment, the number of columns (or rows) is limited to two so that all the connecting leads can be pushed to the outside of the matrix array, hence avoiding the need to provide a guardband (i.e. a width of the tape with no data written onto it) between blocks of tracks from adjacent columns (or rows). In another preferred embodiment, the gaps, instead of being all similar, have different widths and/or lengths to correct the eventual fluctuations in the head to tape positioning.
The matrix may can be covered with a wear resistant material such as diamond-like carbon or any material that would exhibit good tribological properties against the recording medium. The top surface, which corresponds to the side with heads, and the die as a whole, can be advantageously shaped to minimize wear and/or to facilitate the positioning of the tape with respect to the head, both laterally and vertically. This may include, but is not limited to, rounded edges, special features to evacuate loose particles, protruding heads, etc. The area of the die outside the active region where the matrix array is located can also be covered with a material whose properties are well suited against wear, electrostatic charging, or other damaging effects.
Finally, each head of the matrix array is individually driven via at least one independent lead of each head by the control electronics either through individual sources of current, or through a multiplexing system which allows a limited number of heads to be driven at one given time. Each head current can be adjusted independently to the required value to obtain good writing properties, thus enabling the fluctuations due to manufacturing tolerances, lifetime wear and/or tape flutter to be leveled out. Moreover, a special writing sequence of the heads within a row or a column can be chosen so that the crosstalk is minimized.